A photomask may be used to transfer a pattern to a semiconductor wafer. The pattern which is to be transferred onto the wafer may be formed on a photomask substrate which is substantially transparent. The pattern may be formed on a photomask blank structure by standard photolithography processes. Typically, the blank structure is a substrate that is transparent, such as quartz. The substrate may also include thin films of metal or other nontransparent material, which act to block light passing through the substrate. In this way, a pattern is transferred onto the semiconductor wafer. In particular photomasks, the nontransparent material may be a chrome material, such as opaque chrome.
Due to limitations imposed by the wavelength of light used to transfer the pattern, resolution at the edges of the patterns of the photomask degrades. Use of phase-shifting photomasks is one method of increasing the resolution of patterns by creating phase-shifting regions in the transparent areas of a photomask. Standard phase-shift masks generally are formed either by depositing transparent films of appropriate thickness and patterning them over the desired transparent areas using a second level lithography and etch technique, or by etching vertical trenches in the substrate. In both of these instances, the "edges" or "walls" between the phase-shifted and unshifted regions generally result in a transition between high and low refractive index regions.
The characteristics of the phase-shifting photomask described above generally relate to a hard or strong phase-shifter type. This type of photomask is known as an "alternating aperture" or "Levenson-type" phase-shift mask. These types of masks include transmission regions (light transmitted through the substantially transparent regions) on either side of a patterned opaque feature. One of these transmission regions is phase-shifted from the other, and both sides transmit approximately 100% of the incident radiation. These phase-shift regions may be any desired degree, such as 0.degree., 60.degree., 120.degree., or 180.degree., for example. Light diffracted underneath the opaque regions from these phase-shifted regions thus cancels each other, thereby creating a more intense null, or "dark area."
Fabrication of alternating aperture masks is somewhat difficult because the phase-shift layer is defined either by etching the quartz substrate or by depositing a silicon dioxide layer and patterning it. The main limitation of these two techniques is the absence of etch stop or end point during the manufacturing of the phase-shift pattern or during the repair of the phase-shift pattern.
In one prior art method of fabricating a phase-shifting photomask, two transparent layers are deposited above a transparent substrate. These transparent layers are then patterned to create a photomask that has a phase shift of 180.degree. or .pi. radians. It is to be noted that a phase shift of .pi. radians is equivalent to 180.degree., and that degrees and radians may be used interchangeably. The purpose of having two transparent layers over the substrate is to aid in the repair of the photomask. By use of the transparent layers, defects on the surface of the patterns may be minimized.
Other methods of fabricating phase-shifting photomasks are known in the art. For example, a transparent substrate having a metal layer patterned above it may be partially etched multiple times to create a phase-shifting photomask. The purpose of the partial etch steps is to ensure that a defect on the partially etched phase-shifting layer is not transferred to a semiconductor wafer to be exposed by the photomask.